Stent graft delivery systems have been designed to treat abdominal aortic aneurysm (AAA) to minimize the diameter or “French” size of the portion to be inserted into the patient. This usually results in severe compression of very large stents into small diameter tubes or catheters. The drastic inward compression results in high longitudinal forces for loading the stent graft—pushing the pre-compressed stent into the delivery system sheath—and in high deployment forces—occurring when the stent graft is unsheathed at the time of clinical deployment. Other factors cumulatively add to this deployment force including, for example, friction between components of the delivery system handle and the amount of tortuosity in which the sheath is navigated through the patient's vessels.
Deployment accuracy is a term referring to the ability of a physician to choose a target site for stent graft placement within the patient and the ability to “accurately” deliver the stent at the implantation site, the accuracy being measured with respect to both the longitudinal and rotational position of the stent graft. High deployment forces reduce a physician's ability to control deployment accuracy. Other factors can adversely affect deployment accuracy and present additional problems that the physician must address or for which the physician must compensate. These include quality of viewing equipment (fluoroscopy) and rapid blood flow. It would be desirable, therefore, to provide a system that increases stent graft deployment accuracy.
“Pin-and-pull” is a term that has been used in the art to describe many early types of stent/stent graft delivery systems. In pin-and-pull systems, there are two main components: an inner support catheter (e.g., a tube or a rod); and an outer sheath. The outer sheath longitudinally slides over the inner support catheter and can be freely rotated around the inner support catheter (i.e., rotation is independent of longitudinal outer sheath motion). To load a stent graft therein, the inner support catheter is drawn proximally (towards user) so that an interior chamber is created at the distal end of the outer sheath. The stent graft is compressed radially and inserted into this chamber so that the outer sheath houses the compressed stent graft inside its distal end. In this configuration, the inner support catheter prevents the stent graft from moving in a direction towards the physician (proximally) when the outer sheath is retracted. Deployment of the stent graft occurs naturally when the outer sheath is refracted because the individual stents of the stent graft have an outward bias towards their respective fully expanded state.
When the physician is using a pin-and-pull device, the stent graft is maneuvered to the deployment site using fluoroscopy, for example. At this point, the physician is prepared to release the stent graft. The stent graft is deployed in the vessel by “pinning” the inner support catheter relative to the patient and “pulling” back on the outer sheath—thus deriving from these actions the “pin-and-pull” nomenclature.
Because the outer sheath is compressing the stent graft, movement of the outer sheath towards the physician tends to draw the stent graft in this direction. Thus, without the inner support catheter, the stent graft will not be deployed. Minimizing the deployment force allows the sheath to retract with greater ease. It is, therefore, desirable to have the sheath retract as easily as possible.
With high deployment forces, the physician has less control over the placement accuracy. The highest deployment force occurs when the sheath first begins to retract. Once the user has overcome the initial friction between the sheath and the compressed stent, the force then needed for deployment plummets. This rapid decline is almost instantaneous and, often, the physician is not able to react quickly enough to lower the force being supplied to the delivery system. This failure to react results in deployment of more of the stent graft than intended by the physician or in a deployment that fails to hit the intended target site (i.e., low deployment accuracy).
Some mechanisms have been employed to add control to stent graft deployment and minimize this rapid release of stored energy within the delivery system. These mechanisms include screw-type refraction of the stent sheath and/or incorporation of “stops” which prevent inadvertent release of the stent. The screw-type mechanisms slow down the release of the stored energy and help maintain better control of stent release. These screw-type mechanisms also can impart a mechanical advantage by converting the linear force to a torque force. Stop-type mechanisms do not affect conversion or lowering of the deployment force, but help by preventing any over-compensation of the force and any instantaneous release of the force. Neither of these, however, significantly increase deployment accuracy and an improvement in performance would be desirable.
Modular disassociation creates serious type III endoleaks, which can have significant clinical consequences. Creating a mechanical interaction, the modular pull out force will exceed clinical requirements. This type of securement significantly reduces the likelihood of this event. Also, this system does not require rotational alignment between the receiving and inserting components. This makes the mechanism substantially invisible to the doctor and does not add any complexity to the procedure. Further, the system prevents adverse complications during the procedure. By using a proximally facing fold in the graft, there is virtually no chance of accidental ensnarement of a guide wire during the procedure. (If loops or holes were placed in the first member, then a guidewire could potentially get caught without the physician being aware of that ensnarement.) Moreover, the folds in the graft create extra layers of material. Thus, if a securing component were to wear through some of the graft, there multiple layers of the graft will remain to prevent an endoleak. This includes the layer of graft on the inserting member. It is unlikely that wearing of the graft to create an endoleak would occur in both the catheter and catheter direction through three to four layers of material. Significantly, by having multiple engaging members of the second (inserting) stent graft, there is redundancy in the vessel repair system. Therefore, even if some members miss the pockets or even if some members fracture, the overall integrity of the system will still be intact. Further redundancy in the vessel repair system is present by providing multiple sets of folds in the first component. These folds can be at the very end of the stent graft as well as multiple folds moving up the length of the stent graft. This configuration and variants thereof can cover any leg prosthesis stent graft.
Thus, there is a need to develop new, useful and effective delivery systems, components and methods to treat AAA.